This invention relates to a high precision laser interferometer. Laser interferometers are used to measure the movement of precision positioning tables, for example.
Generally speaking, an interferometer is an instrument that measures distances and very small changes in distances by means of the interference of two beams of light. In a laser interferometer, the light source is a laser tube providing a strong coherent monochromatic light beam. In the well known Michelson interferometer, a source of monochromatic light is split (by a beam splitter) into a reference beam and a measurement beam. The reference beam is directed along a fixed path (reference arm) to a light sensor (photodetector). The measurement beam is directed to follow a path that varies with the distance being measured (measurement arm). The two beams are recombined at the beam splitter so as to interfere. The resulting interference is monitored at the sensor. When there is a phase difference between the two recombining beams due to the difference in distances traveled by the beams in the reference and measurement arms, the beams interfere with each other reducing the intensity of the measured signal. The output of the sensor will reach a positive or negative peak each time the distance traveled by the measurement beam increases or decreases by one half wavelength of the monochromatic light being used. The polarity changes are counted to provide a measure of the change in distance.
In the well known double frequency Doppler shift interferometer, the reference beam and measurement beam have frequencies that are slightly different. The changing distance traveled by the measurement beam causes the frequency of the measurement beam at the sensor to change (the Doppler shift). The difference between the source frequency and the Doppler shifted frequency is integrated over time to provide the change in distance itself.
The need for a low cost laser interferometer which is stable over a long period of time in industrial environments is known. Stability in a laser interferometer means that over a long period of time the measurement of a standard distance does not deviate more than an allowed fraction. The search for yet greater and greater stability has led to instruments of greater and greater complexity.
The lack of stability in a Michelson interferometer may be related to the lack of pointing stability in the laser beam and in the lack of mechanical stability of the remainder of the system due to temperature changes in the structure holding the measurement and reference arms. Fortunately, the stability of the frequency and power outputs of the laser itself is not a limiting factor. Extremely stable single-frequency lasers are now available at a reasonable cost. See, for example, U.S. Pat. No. 4,819,246. The stability of laser wavelengths are measured in parts per million per year, say 0.02 to 0.1 ppm/year.
Mechanical instability in the Michelson interferometer structure has the effect of changing the effective lengths of the beam paths as well as changing the degree of overlap and the angles of convergence of the reference and measurement beams at the sensor. Imperfect overlap of the reference and measurement beams at the sensor results in only one or the other of the beams striking a portion of the sensor. Without interference, this portion of the sensor simply produces a DC component in the sensor output. Mechanical instability can change the degree of overlap and therefore the DC component of the sensor output. Since the AC (sinusoidal) component of the interference signal is squared in a single level detector, any drift in the DC component of the signal will change the duty cycle of the squared signal and a sufficient instability will result in loss of the AC component altogether.
As a practical matter, the two beams cannot be made to arrive at the sensor perfectly aligned. Hence, a fringe pattern strikes the sensor where the two beams overlap. The fringe pattern may be considered the result of two collimated beams approaching the sensors at different angles or two divergent beams approaching the sensor along spaced axes or both. If the spacing of the fringe pattern relative to the size of the sensor is insufficient, more than one fringe may strike the sensor at all times. Since the output of the sensor is related to the sum of the intensity of the light over the entire surface of the sensor, the output signal is related to the total number of fringes and factions thereof that strike the sensor. The AC component of the output signal of the sensor can be lost altogether when more than a single fringe strikes the sensor. Decreasing the angle of the convergence of the two beams and/or their spacing, if the beams are divergent, the fringes become more widely spaced. Unfortunately, as will be explained in detail, this actually results in increased instability.
Elaborated systems have been devised to overcome the above-noted problems. For example, the double frequency Doppler shift interferometers measure frequency shifts between a measurement and reference beam thus making them insensitive to fringe contrast and light source intensity. See, for example, U.S. Pat. Nos. 3,788,746 and 3,714,607. Unfortunately, Doppler shift laser interferometers can lose count at speeds easily attained by modern x-y position stages.
Another system based upon the Michelson interferometer uses a single frequency and a complex detection system detecting three different interference signals, one in quadrature with the other two. The three signals are combined in a way to cancel the effects of thermal drift on the DC level of the interference signal. See U.S. Pat. No. 4,360,271.
With an interferometer, two signals in quadrature (90.degree. out-of-phase) must be detected to enable discrimination of the direction of change of the measured distance. Typically, the reference and measurement beams are both split and recombined as two different interference patterns at sensors spaced one quarter of a fringe so that the output signals at each sensor are in quadrature.